May 22, 2007

Volcanic eruptions preserve ancient history for Croft

Case assistant professor harvests the fossilized bones of animals from South America

A catastrophic mudflow some 25 to 28 million years ago stopped a nine-inch armadillo in its tracks.

Frozen in time, the small animal would not come to light again until this past March when Darin Croft from the Case Western Reserve University School of Medicine—a member of an international field team including U.S. and Chilean researchers—picked up the fossil embedded in the dark purple rock that was once volcanic mud.

The yet undescribed specimen was found in a remote central Chilean field site in the Río Maipo valley.

The small animal resembles a modern-day armadillo with its hinged plates that form a hard armored shell. It is small by today's standards, but its fused limb bones indicate the animal had reached adulthood.

Traveling by truck and by foot to an elevation of approximately 2,000 meters, Croft describes this new site upon arrival as having "skulls everywhere." Such specimens are typically very rare.

According to Croft, the animals found at the site suggest it is younger than some of the other localities they have discovered. Since 1988, the team's central Chilean field work has radiated out to as far north as Río Colorado and as far south as Laguna del Laja (though other sites have also been discovered in the extreme north and south of the country). The sites range in age from 45 to 15 million years old.

The first locality discovered (Tinguiririca) has turned out to be one of the most important; it demonstrates that grasslands were present in South America nearly 35 million years ago—much earlier than in other continents. Croft, who is an assistant professor of anatomy at the medical school with a secondary appointment in the biology department in the College of Arts and Sciences at Case, provided some of the strongest evidence for the grassland interpretation through his detailed studies of Tinguirirican mammals. As a paleoecologist, he reconstructs past habitats by studying the shape, size, and composition of teeth, jaws, and limb bones; these remains provide evidence about what animals ate and how they moved.

The latest find is among some 1,800 specimens, including many new animal species, that have been collected from the group's numerous fossil-rich central Chilean localities. The other members of the research team include John Flynn (American Museum of Natural History, New York), André Wyss (University of California—Santa Barbara), and Reynaldo Charrier (Universidad de Chile).

Croft joined the team in 1998 as a doctoral student at The University of Chicago and The Field Museum; Flynn was working at the Field Museum at that time and served as Croft's Ph.D. advisor. The researchers have brought back so many specimens that it would take several lifetimes to describe and name all that has been found.

This past field expedition also returned the first batch of studied fossils to the National Museum of Natural History in Santiago, Chile, where a temporary exhibit was held to highlight the group's work. Prior to the discovery of these new fossil localities, virtually nothing was known about Chile's fossil history between 45 and 15 million years ago.

These fossils provide much new information about the evolutionary history of South American's unique fauna that developed and proliferated after the land mass lost connections to Africa, Australia, and North America. The continent gave rise to notoungulates, a diverse group of hoofed animals that resembled horses, rhinos, antelopes; marsupials, including many types of opossums; and great radiations of monkeys and rodents.

Some of the Chilean fossils have proved to be especially important in determining that these last two groups originated in Africa and somehow came to South America more than 35 millions years ago (perhaps on rafts of floating vegetation spewed into the ocean by storms).

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